This is the first step towards replacing the implementation of
attribute sets with an absl::btree_map.
Currently many access are done using array offsets and pointer
arithmetic, so this change is currently causing Nix to fail in various
ways.
Meson is unable to use CMake in Nix to determine the internal
structure of the Abseil libraries.
This commit adds an explicit list of most of the Abseil targets that
are relevant (so far) and bundles them into a list that is linked
together.
1. First of all, this doesn't work in nixpkgs. Per [1], gcc ignores `-L`
for purposes of `--print-file-dirs`, which breaks horribly on linux. But
if we don't pass extra dirs, meosn first just tries `-l...`, which does
work.
2. Even if it did work, `libdir` means where we are installing libs, not
where libs are expected to be found. Those are not necessarily the
same (again, nixpkgs), and even when they are and non-standard, it is
better to use DESTDIR or have a modified toolchain.
[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87758
(cherry picked from commit a142164e746644e20f66908c156ca913bef4664f)
